As a packaging material for holding a content such as retort pouch foods including retort pouch curry, refills including shampoo refill and liquid detergent refill, and electricity generating elements for a secondary battery, wide use has been made of a composite multi-layer film comprising an aluminum foil with a layer of a polyolefin film layer on one side of the aluminum foil and a layer of a polyamide on the other side (see, for example, the following Patent Document 1). Polyolefin has a heat adhesion property and, therefore, a packaging material with a polyolefin film inside can be heat-sealed. Polyamide is superior in mechanical strength and, therefore, can increase durability of a packaging material.
A polyolefin film has conventionally been adhered to an aluminum foil via an anchor coat agent or an adhesive. Therefore, such a packaging material cannot hold a liquid that has such a strong permeating or solubilizing ability as to damage the anchor coat agent or the adhesive.
In addition, use has conventionally been made of a non-surface-treated aluminum foil. Therefore, a packaging material with such an aluminum foil cannot hold a liquid that damages aluminum, even if the liquid does not affect the adhesive layer.
Further, many researches have been made on lithium ion secondary batteries and electric double-layer capacitors as batteries for hybrid automobiles and electric cars. Conventionally, an electricity generating/storing element has been sealed in a metal vessel in lithium ion secondary batteries and electric double-layer capacitors. Due to the severe fuel efficiency standards imposed on automobiles, weight reduction of automotive parts has been pursued. It is very promising for weight reduction of automotives to use the composite multi-layer film as an exterior packaging material for secondary batteries.
However, organic solvents used as an electrolyte in an electricity generating/storing element for lithium ion secondary batteries and electric double-layer capacitors have a strong permeating or solubilizing ability and, therefore, damage the anchor coat agent, adhesive or aluminum. Therefore, the use of the above-mentioned composite multi-layer film results in a lithium ion secondary battery or an electric double-layer capacitor with less reliability.
In order to overcome these problems, a method called chromate treatment is widely known in which the surface of an aluminum foil is treated with chromic acid-type chemicals (see, for example, Patent Document 2). This treatment forms a coating on the surface which coating significantly increases corrosion resistance of the aluminum foil and allows the aluminum foil to adhere to a polyolefin film by heat lamination without any anchor coat agent or adhesive. However, this technique is accompanied with a serious problem of using chromium which is an environmental load substance.
Chromium-free chemical conversion treatments such as boehmite treatment (hydrothermal treatment) and phosphate treatment are known as a method for treating the surface of aluminum without chromium. These treatments attain corrosion resistance similar to that in the chromate treatment. However, if a polyolefin film is adhered to an aluminum foil subjected to chromium-free chemical conversion treatment by heat lamination without any anchor coat agent or adhesive in such a temperature range that the polyolefin film can be laminated to the aluminum foil without melt-adhering onto a processing machine, sufficient adhesion strength cannot be obtained between the polyolefin film and the aluminum foil.
Further, a composite multi-layer film needs to have a high low-temperature resistance when applied to a lithium ion secondary battery or an electric double-layer capacitor for an automobile battery. In the prior art, crystalline polypropylene has been used as the polyolefin film, taking much account of heat resistance. This results in unsatisfied low-temperature resistance. In order to improve the low-temperature resistance, it has been widely carried out to use a block copolymer as the crystalline polypropylene or to add rubber to the crystalline polypropylene. However, in that technique, such morphology appears that “islands” of the rubber as a modifier for the low-temperature resistance are dispersed in a “sea” of the crystalline polypropylene. This may cause permeation of liquid through the “islands”. That is, organic solvents used in lithium ion secondary batteries and electric double-layer capacitors are at high risk of leakage in a long term, because they have a strong permeating or solubilizing ability.    [Patent Document 1] WO99/40634    [Patent Document 2] Japanese Patent Application Laid-Open 2003-288865